Thermit compositions consist of a mixture of finely divided oxidisable metal together with an oxidising agent therefor, usually consisting substantially or wholly of iron oxide. When such a composition is ignited, a highly exothermic reaction takes place with oxidation of the oxidisable metal and reduction of the iron oxide to molten iron. The oxidisable metal of choice is aluminium and a wide variety of thermit compositions is known which in addition to aluminium and iron oxides contains other modifying agents such as refractory fillers which slow down the speed of the reaction, and initiators such as fluorides which assist positive ignition of the whole of the mass of composition used.
In the casting of molten metals to form ingots, it is customary to pour molten metal into an ingot mould from a substantial height. The molten metal falling on the base of the mould, or on a separable bottom plate constituting the base of the mould, is very hot and the base or bottom plate is accordingly subjected to substantial erosive forces which wear a depression in the mould or bottom plate. Because the base of the mould wears much faster than the remainder, it is common practice to form the mould from a mould body superimposed on a substantially flat base plate which can then be scrapped when too eroded for further satisfactory use. Although separating mould body and base plate in this way leads to economies, even base plates themselves are expensive and accordingly it is desirable to prolong their life e.g. by repairing eroded areas. Mechanical methods such as welding steel plates and the like over eroded areas have been tried but these are difficult and time consuming to apply. Thermit compositions have also been proposed which may be applied to the eroded area and ignited there. They heat up and then cool to leave a solidified mass in the eroded area. Such known compositions have not proved very satisfactory in use. Many are difficult to apply, give off much fume when ignited, which is not only inconvenient but may constitute a health hazard and additionally do not lead to satisfactory repairs.